Surface discharge plug for low tension and condenser discharge



May 15, 1956 w. B. SMITS 2,745,980

SURFACE DISCHARGE PLUG FOR LOW TENSION AND CONDENSER DISCHARGE FiledMarch 3, 1952 INVENTOR M725 5292 .snwzs,

BY QLEAERQ ATTORNEYS United States Patent SURFACE DISCHARGE PLUG FOR LOWTENSIGN AND CONDENSER DISCHARGE Wytze Beye Smits, Voorburg, Netherlands,assignor to Smitsvonk N. V. Research Laboratory Comp., Rijswijk,Netherlands Application March 3, 1952, Serial No. 274,607

1 Claim. (Cl. 313-431) The invention relates to a surface dischargespark plug for low tension and condenser discharge. Spark plugs of thistype are known, for example from the Netherlands patent specificationsNos. 46,114, 55,918 and 62,644. Further, such spark plugs have beendescribed in an article by W. Bye Smits in the periodical De Ingenieur,63, No. 3, p. V. 1/9 (January 19, 1951), entitled: The Smitsvonk LowTension Capacity Ignition.

Such spark plugs normally have a flat operative surface, which comes tolie approximately on a level with the wall of the combustion space.

It has been found that in some cases these known spark plugs do notoperate quite satisfactorily, more in particular not in poorly scavengedtwo-stroke engines. Especially many small two-stroke engines areinsufl'iciently scavenged, for example because the crank-case with thebottom of the piston is used as scavenge pump.

Now it has appeared from experiments that a better ignition can beattained in such cases when according to the invention one or morechambers are formed in the spark plug, in the side wall of which arelocated the operative parts of the electrodes, near the mouth of thechamber or chambers.

It is not certain to what cause this favourable effect is to beattributed. It is, however, probable, that in poorly scavenged engines alayer of combustion gases adheres to the walls of the combustionchamber, thus remaining in the neighbourhood of the spark plug. Thislayer will naturally expand and be compressed again upon expansion andcompression respectively in the cylinder, but the layer will remainsubstantially present. The surface discharge sparks for the ignitionwill now therefore operate, at least partially, in gases which arecombustion gases and which do not contain suflicient amounts of stillcombustible particles and air to start the combustion of the mixtureefiiciently and rapidly. The consequence of this is misfiring andirregular operation of the engine. When one or more chambers, areprovided according to the invention, the layer of combustion gas in theneighbourhood of the spark plug is probably forced, at least partially,into the chamber or chambers during the compression stroke. A quantityof foul gas thus accumulates in the deepest part of the chamber orchambers, as a result of which the layer of combustion gas present nearthe operative parts of the spark plug will disappear altogether or atleast be greatly reduced in thickness. It is therefore essential thatthe operative parts of the electrodes are near the mouth of the chamheror chambers, since otherwise they would operate in the foul gas. This isin agreement with what was observed during experiments. It is, however,also possible that during the scavenging of the cylinder the chamber orchambers, owing to the interruption of the smooth surface of thecombustion space, cause a vortex in this place when the scavenging-aircurrent passes by, said vortex sufliciently rarefying any foul gases inthe neighbourhood of the operative parts of the electrodes, and saidvortex being maintained during the compression stroke, so that thecombustion is started and propagated in a particularly fool-proofmanner.

Although applicant suspects that the first-mentioned effect is the mostlikely explanation of the advantage obtained, it is not to be consideredimpossible that the two actions occur simultaneously and make theirjoint favourable influence felt.

The invention will now be explained more in detail with reference to twoembodiments shown in the drawing.

Fig. 1 illustrates a spark plug according to the invention with a singlechamber, in which the operative parts of the electrodes are provided inthe side wall of the chamber, one part of the figure being a front Viewand the other part an axial cross-section.

Fig. 2 illustrates an embodiment in which the opera tive parts of theelectrodes are provided in the top surface of the spark plug, in theside wall of the single chamber. This Fig. 2 only shows a part of aspark plug.

As shown in Fig. l, a spark plug according to the invention as usual,has a casing 1, connected to the ground, and insulating body 2, and aconnecting terminal 3. The casing 1 has a screw-threaded section 4, bymeans of which the spark plug can be screwed, for example, into acylinder cover or the like, electric contact with the ground beingestablished via the screw-thread.

As shown in cross-section in Figs. 1 and 2, two electrodes 5 and 6 arelocated inside the screw-threaded section 4 of the casing 1, the saidscrew-threaded section 4 forming the connection with the electrode 6.The space between the electrodes 5 and 6 is filled up entirely orpartially with a separator body 7 of only slightly conductive orsemi-conductive material. In both spark plugs a chamber 8 has beenprovided, which is located coaxially in the spark plug and iscylindrical. This chamber has a depth which is greater than the diameterof the cylinder.

In the embodiment according to Fig. 1, in which the operative edges ofthe electrodes 5 and 6 lie in a circle along the side wall of thechamber 7, these edges are at a distance from the mouth of the chamberwhich is less than one third of the depth of the chamber.

In the embodiment according to Fig. 2, in which the operative edges ofthe electrodes are located in the top surface of the spark plug in theside wall of the chamber, these operative edges preferably lie in aconcentric circle round the chamber 7. In a spark plug according to Fig.2 the action will probably not be inferior to that in the embodimentaccording to Fig. 1, but a spark plug according to Fig. l is easier tomake.

The invention may be embodied in several other ways as well. A number ofchambers may be provided, each of these, for example, having operativeparts lying concentrically around them. A number of chambers may alsopossess jointly a set of operative edges of electrodes.

Apart from the top surface, the chambers might also be provided in aside surface of the spark plug. Thus a construction is formed such asshown, for example, in Fig. 7 of the above-mentioned article in DeIngenieur, with the difierence that the operative edges of theelectrodes are provided near the mouth instead of in the deepest part ofthe chamber.

I claim:

'Surface discharge spark plug arrangement for low tension and condenserdischarge, comprising: means forming a chamber having its opening at thesparking end of said plug, said chamber consisting of a vaulted bottomand, extending from said bottom to said opening, unobstructed parallelsidewalls; sparking electrodes disposed intsaidt sidewalls forming.alpart thereof, said elec:

trodes. lying at a. distance, less than. one-.third the, depth. of thechamber from the. chamberopening, and; insulating materialformingaspa'rkingsuriace disposed between said electrodes.

References Cited UNITED STATES PATENTS in the file of this patentStephenson June 20, Brewster Mar. 6, Burhans Sept. 23, Mitchel Sept. 15,King Nov. 18, Mitchel Aug. 15,

